1. Field of the Invention
This invention relates to magnetic disk drives and more particularly to the storage of information on a magnetic disk.
2. Description of the Related Art
Hard disk drives (HDD) are magnetic data storage devices on which user created data can be written and read. Referring to prior art FIG. 1 there is shown a schematic drawing of such a HDD (11) on which only a single disk (1) is mounted for clarity or the illustration. Typically such a drive would contain one or more flat disks (1) which rotate at high angular velocity about a central spindle (12) and on which data is encoded by magnetic read/write heads (13) that move above the surface of the disk and are radially positioned by motor driven actuator arms, only one of which is pictured here (14).
The read/write head (13) is mounted on the distal end of the actuator arm. Details of the mounting structure are not shown. By a rotation of the actuator arm about a pivot (16), the head can be made to read and write on any one of a plurality of narrow, concentric annular tracks (an exemplary track being indicated here as a single dashed line (17)) on the rotating disk. Under ideal circumstances the data written by the read/write head forms these narrow annular circular tracks, thinner than 0.3 microns, that are concentric with the rotation axis of the spindle. The dark rectangular segments (15) are schematic representations of radially aligned azimuthal servo data segments shown as intersecting the circular tracks. Although only one track (17) is shown here, these servo data segments would normally extend through all the tracks and they would contain encoded information that enables the head to locate particular tracks and positions on the tracks.
The disks in HDD's are formatted (which may occur prior to or subsequent to mounting) so that during operation the magnetic read/write head can be properly positioned to record user data and then later to locate that data and read it back. As illustrated schematically in FIG. 2, formatting effectively divides each disk surface (1) into concentric numbered annular “servo tracks” (22, 23, 24), which are formed so that the servo mechanism of the HDD (the electro-mechanical mechanism that controls the actuating process) can locate the proper radial position of the track containing the user data. The servo tracks (22, 23, 24) are subdivided azimuthally into alternating substantially wedge-shaped sectors (31, 41) (32,42), one sector (the data sector) being reserved for user data storage (41), the adjacent sector (31) (the servo sector) containing servo data that is used to locate the user data and maintain the read/write head positioned accurately along the center line of the track (two dashed line segments (50) and (51)). Typically, the servo data within a servo track comprises a servo data field that identifies the track, a servo “burst” that is used to properly align the head with the track (typically the track centerline) and other fields for read/write and system operation. The user data can be overwritten, but the servo data remains fixed. The tracks so formed are characterized by a “width” (70) and a “pitch” (60), the width being the distance between the radial edges of the track and the pitch being essentially the radial distance between centerlines of adjacent tracks. It is to be noted that, for simplicity, the tracks schematically indicated in FIG. 2 show the servo and data portions as having the same common annular width along the entire track. In the present invention, however, these widths will vary, as will be shown in FIG. 4 described below. The region between the outer and inner circumference of the tracks is denoted the data band.
As the disk rotates, the head reads positional information from the servo sectors and the radial position of the head is continually adjusted to place it correctly at the position at which it is to read or write data. If the actual location of the head (as determined by the servo data at its location) does not coincide with its target location (the servo data at the position it is intended to reach) a position error signal (PES) is generated that causes the actuator to reposition the head. As a result of this entire process by which the writing of user data is guided by servo track data that is already radially and azimuthally encoded, the user data is written in radially concentric tracks called data tracks. The radial spacing (pitch) of the data tracks need not be the same as that of the servo tracks, but the ratio of one spacing to the other is generally constant across the disk surface.
Because the head-to-track angle changes as the head moves across the disk (and for other reasons as well) the minimum acceptable track spacing for data tracks varies across the disk. Traditionally, a single constant track spacing is selected that maximizes (within the constraint of a single spacing) the amount of data that can be stored and reliably read back. Alternatively, manufacturers can vary, or “warp” track spacing across the disk in order to store more total data on each disk surface.
At present, data track spacing has been warped by varying the spacing of the servo-tracks across the disk. In this way the consistent relationship between the spacing of the data tracks and the spacing of the servo tracks is maintained. The ratio of the data track to servo track spacing could be 2/3, 1/1, or some other ratio, but it is fixed across the surface of the disk.
Various approaches to defining track spacings can be found in the prior art. Kagami et al., (US Published Patent Application 2005/0041322) describes a disk that is partitioned into radial zones, each having some given radial width. Each zone has the same servo track pitch. In fact, the servo track pitch is constant over the entire disk. However, in each zone, the ratio between the data track pitch and the servo track pitch can be made to change. This is done by defining a numerical coefficient for each zone and then instructing (by a stored “firmware” program) the recording head, when it is in that zone, to locate its data track at a distance from the servo track location that is given by the product of the coefficient and some given constant ratio. Kagami also notes that the disk can also be divided into zones but it is the actual servo track pitch that is made to be different in each zone. Then, the ratio of the data track pitch to servo track pitch is maintained as a constant. Therefore, it is the actual variable servo track pitch in each zone that controls the data track pitch in the zone.
Ikeda et al. (US Published Patent Application 2004/0201914) describes a method for deriving servo track pitch from the read/write element offsets. In this situation the disk is literally customized for the relationship between a particular read and write head.
Chiao et al. (US Published Patent Application 2004/0136104) describes a disk drive with a fixed servo track pitch but a variable data track pitch.
Allen et al. (U.S. Pat. No. 6,947,248) describes a firmware program that can calculate variations in spacing between the read and write elements of a magnetic head and, thereby, change the (fixed) ratio between servo and data track widths from head to head in a multi-disk cartridge.
Liikanen et al. (U.S. Pat. No. 6,256,160) describes a disk in which the servo track pitch and the data track pitch are unequal and, with a particular region of the disk, are related by a constant factor defined as the TPI scale factor.
Emo et al. (U.S. Pat. No. 6,005,725) describes a multi-disk HDD in which the recording zone boundaries on each disk are optimized to the characteristics of the read/write head associated with that disk. In this invention, the data track width is determined by the track width of the read/write head and servo and data tracks have the same width and pitch.
Emo et al., in related application Ser. No. 09/501,711, describe a method of manufacturing a disk drive in which servo track data is written onto a disk, the track width performance of the associated read/write head is measured and the data track pitch is thereby established. As distinguished from U.S. Pat. No. 6,005,725, this invention permits servo track width to be separately specified from data track width, while the data track width is determined from actual track width of the read/write head. The servo track width and pitch is constant across a disk.
The above prior art either defines pitch ratios to optimize a particular head performance or allows pitch ratios to vary within fixed zones. There is, however, another method by which warping can be accomplished. Data track spacing can be warped as data is being written, if the ratio of data tracks to servo tracks is varied on a track-by-track basis. This method is denoted by us as “soft warpage” and it can be considered as a continuous and smooth form of warping the pitch ratios. In this method, the average track spacing and warpage can be varied independently, even after the disk has been formatted. Since optimum data track spacing changes smoothly across the disk, the result of soft warpage could be data tracks written with offsets from the servo tracks that change smoothly from track to track.
To support the offsets between the data tracks and servo tracks an algorithm must be developed and added to the HDD firmware to determine and control the location of each written data track. The inventors believe that this approach offers many advantages relative to approaches taken within the prior art. These advantages include the capability of smoothly and continuously varying the density of written data across the surface of a disk, the capability of varying the total data capacity of each disk among a plurality of disks, and a capability of experimenting with different types of disk formatting during the manufacturing process so as to improve the efficiency of that process.